1. Field of the Invention
The present invention relates to a zoom lens assembly and more particularly a zoom lens assembly provided with an improved focusing mechanism.
2. Description of the Prior Art
U.S. Pat. No. 4,439,019 issued on Mar. 27, 1984 disclosed a zoom lens provided with a focusing ring and a zooming ring in which continuous focusing from infinity to nearest object distance is possible by an operation for rotating the focusing ring. This prior art zoom lens has advantages of simple construction and easy operation. In order to maintain the optical performance of the lens during focusing the focusing lens group is moved as a whole while changing the separation between lens elements of the focusing lens group. No particular close-up (macro-photography) mechanism is needed.
However, the prior art zoom lens, in particular, such a zoom lens having large magnification change with zooming, has some difficulties in maintaining the optical performance perfectly. Aberrations cannot be corrected sufficiently by the focusing lens group. Especially in the focus position for near objects it is difficult to adequately correct the aberrations.